#7 and #9 are the "ano machi" stories where Oikawa and Iwaizumi are finally together. The period with Matsun was before #7 and #9 when Oikawa ran away from everything (after severely damaging his knee and withdrawing from Nationals) including geting away from Iwaizumi. I'm not sure what fanbook number that was. I think it's #13 or #15. But Oikawa was so fucking sad and desperate, he just went with flow. Aside from Matsun pity fucking Oikawa, i think Matsun liked Oikawa a bit that's why he helped create an escape route for Oikawa. This series literally burned my heart. I'm just so happy right now. As far as the story goes chronologically, Oikawa is with Iwaizumi now.
Did u guys know gusuri has a 3rd work for this series? It's the period where oikawa already left iwaizumi and is hiding from him. During that 4 years, he had a sexual relationship with matsun. Lol. The series if very poignant af